The present invention relates generally to semiconductor devices and, more particularly, to the use of transition metal boride films as diffusion barriers in devices such as gate stacks and digit line stacks.
In some semiconductor memory circuits, word lines, which are formed from a uniformly-thick conductive layer, form both gate electrodes and gate interconnections. Whenever a word line passes over a field-oxide region, it functions as a gate electrode interconnection; whenever the word line passes over a gate dielectric layer overlaying an active region, it functions as a gate electrode.
In early generations of integrated circuits, gate electrodes and electrode interconnections were often etched from a heavily-doped polycrystalline silicon (polysilicon) layer. To achieve increased operational speeds and lower stack heights in subsequent generations of circuits, it was necessary to decrease the sheet resistance of the conductive layer from which the gates and gate interconnections were formed. Recently, the use of pure metal layers formed from materials are being investigated to enhance the conductivity of the polysilicon transistor gates and gate interconnections. Tungsten (W), for example, is of particular interest because it is relatively inexpensive, has a high melting point, and is compatible with current circuit manufacturing processes. Thus, low pressure chemical vapor deposited (LPCVD) tungsten silicide (WSix) is being investigated in the fabrication of polycide gate structures to form low resistance word lines in semiconductor devices such as dynamic random access memory (DRAM) cells.
As illustrated in FIG. 1A, a wafer includes a semiconductor substrate 10 which may include one or more previously formed layers or active regions. A gate dielectric such as a silicon oxide layer 14 is deposited or grown over the surface of the substrate, and a gate stack 22 is formed over the silicon oxide layer. The gate stack 22 includes a gate polysilicon layer 16 which helps improve the adhesion of a subsequently deposited tungsten silicide film. The gate stack also includes a tungsten silicide layer 18 deposited, for example, by LPCVD over the gate polysilicon layer 16. The polysilicon and tungsten silicide layers 16, 18 are patterned and etched using conventional photo-lithographic techniques to form the polycide gate electrodes. Ion implanted source and drain regions 12 are formed, and the wafer is subjected to an annealing process at an elevated temperature.
WF6 and SiH4 are among the reaction gases typically used during the deposition of the tungsten silicide film 18, and, therefore, fluorine atoms generally are incorporated into the tungsten silicide film 18. When the polycide structure is subsequently annealed at high temperatures, fluorine atoms tend to diffuse through the gate polysilicon 16 into the gate silicon oxide layer 14. The fluorine atoms react with the oxide and break the Sixe2x80x94O bonds to replace the oxygen at those sites. The released oxygen diffuses to the interface of the SiO2 layer 14 and oxidizes the silicon and polysilicon resulting in an increased oxide thickness 20 (FIG. 1B). The additional oxide can cause device degradation, such as a shift in the threshold voltage and a decrease in the saturation current.
Attempts have been made to reduce the diffusion of fluorine into the gate silicon oxide layer by forming a thin film conducting diffusion barrier between the tungsten silicide film 18 and the gate oxide 14. For example, diffusion barriers of materials such as titanium nitride, tantalum nitride and titanium tungsten have been proposed with some success. Nevertheless, room remains for improvement in structures such as gate stacks as well as digit line stacks, among others.
In general, techniques are disclosed for fabricating semiconductor devices and integrated circuits incorporating a transition metal boride layer. The transition metal boride layer can act as a diffusion barrier to improve the properties of the device.
For example, according to one aspect, a method of fabricating a semiconductor device includes forming a transition metal boride layer on a layer comprising silicon and forming a conductive layer on the transition metal boride layer. The transition metal boride layer can be formed by various techniques, including chemical vapor deposition. In other implementations, a transition metal layer is formed on the layer comprising silicon, and the transition metal layer is exposed to a gas containing boron. Rapid thermal processes as well as plasma treatments can be used to expose the transition metal layer to the boron-containing gas, thereby forming the transition metal boride layer. Alternatively, the transition metal layer can be implanted with boron ions.
According to another aspect, an integrated circuit includes a substrate, a gate dielectric disposed over the substrate and a gate stack disposed on the gate dielectric. The gate stack includes a layer comprising silicon, such as a polysilicon layer, a transition metal boride layer disposed on the layer comprising silicon, and a conductive layer disposed on the transition metal boride layer.
According to yet another aspect, an integrated circuit includes a substrate, a polysilicon layer disposed over the substrate, and a digit line stack disposed on the polysilicon layer. The digit line stack includes a transition metal boride layer disposed on the polysilicon layer and a conductive layer disposed on the transition metal boride layer.
Various implementations include one or more of the following features. The transition metal boride layer can include a material selected from the group consisting of zirconium boride, titanium boride, hafnium boride and tantalum boride. In some implementations, the transition metal boride layer has a resistivity in the range of about 5 to 150 microOhms-centimeter and a thickness preferably less than about 200 angstroms.
The conductive layer can be formed by a process using a reaction gas comprising fluorine. In such cases, the transition metal boride layer can function as a diffusion barrier layer to help reduce or eliminate the diffusion of fluorine atoms from the conductive layer into the polysilicon layer and into the gate dielectric during subsequent processing. Accordingly, the thickness of the dielectric layer does not increase as a result of subsequent annealing or other processes performed at an elevated temperature. The transition metal boride layer also can function as a diffusion barrier layer to reduce diffusion of silicon atoms into the conductive layer. Low resistance can be maintained and high temperature stability can be achieved so that little or no degradation of the device results.